The Panino al Prosciutto is the raw-cured ham sandwich, and the thing that defines it is that the meat was never cooked. Prosciutto crudo is a pork leg salted and air-dried for months or years until it is dense, deeply savoury, and faintly sweet, sliced so thin that light passes through it. This is a different animal from the cooked-ham panino entirely: where prosciutto cotto is pale, mild, and gently brined, crudo is concentrated, salty, and assertive, a meat that has spent a long time becoming intense rather than a short time staying tender. A few slices of it on the right bread, and nothing that would mask it, is the whole sandwich.
The craft is the slice and the bread under it. A good crudo is cut almost to transparency and laid in loose, airy folds rather than flat sheets, so that air moves through it and it reads as delicate on the tongue instead of as a salt-heavy slab. Because the meat is fatty and intense, the bread is chosen plain and structured, often a crisp-shelled roll or a length of unsalted Tuscan loaf whose neutrality lets the cure carry. Butter appears only where it bridges a very lean leg to a dry crust, and even then sparingly. The pairing is regional and not casual: the sweet Parma wants one kind of bread, the more savoury San Daniele another, the firm Toscano the unsalted pane sciocco of its own hills.
The variations are the entire tradition of Italian raw-cured ham, and each is its own subject rather than a line here. Prosciutto di Parma, San Daniele, the wild-boar prosciutto di cinghiale, the rustic prosciutto toscano: each is a distinct cure with its own salt, sweetness, and bread match. The cooked counterpart, milder and entirely its own thing, is the panino al prosciutto cotto. Each of these is one cured leg given the bread it belongs with, and each deserves its own article rather than being crowded in here.